In many situations, it is desirable to determine the location and movement of a person or object within a building or relatively well-defined space, such as a home, hospital, or prison. For example, elderly or infirm persons living alone or in a nursing home may need frequent or even relatively constant monitoring by caregivers, and firefighters in a risky building may also need to be tracked.
Some advanced techniques, such as radar, GPS, Time-of-Flight, and Time-Difference-of-Arrival may be used to determine the location and movement of persons or objects. For instance, radar may be used for tracking a person or object, in which an electromagnetic radiation is reflected off the person or object. GPS may also be used for tracking a person or object, in which the person or object may be equipped with a GPS transmitter for transmitting signals which can be received by GPS satellites. However, these techniques require relatively expensive equipment, and might not work effectively under certain circumstances. For example, both radar and GPS systems would not work effectively within a building, because the walls and ceilings of the building would block radar signals as well as block access to GPS satellites.
Relatively less expensive radio technique may also be used to determine the location and movement of a person or object by applying the property that the power level of a radio signal attenuates at a logarithmic rate over distance. Given the signal power level transmitted by a radio transmitter, a radio receiver can measure the received signal power level, and can calculate a distance estimate between the radio transmitter and the radio receiver. Using several radio transmitters at different known locations, a mobile radio receiver is able to determine its own location using trio metric or multilateration calculations. However, this approach based on signal power attenuation will produce an accurate location estimate only in a perfect world devoid of other radio interference or other physical objects absorbing or reflecting radio signals. Since radio signal power level attenuates at a logarithmic rate, the accuracy of distance calculation deteriorates exponentially as the signal power level decreases, thus the accuracy of the estimated location using this approach is significantly affected.